Add support for custom user model in AuthKitAuthenticationRequest

[zachgilbert97]

Description:

• Currently the WorkOS Laravel integration expects the App\Models\User to represent a user, but some applications may use a different model.
• Through the Laravel config you typically define an alternative user model, under auth.providers.users.model (defaulting to \App\User\Model or reading from AUTH_MODEL in the .env)
• As such, the AuthKitAuthenticationRequest here is updated to take the model from config, and typed to expect a model implementing the Authenticatable contract
• This makes the dependency more versatile to Laravel applications that do not use the convention of \App\Models\User being the user model.
• Existing Laravel applications which do use the \App\Models\User as the user model will not break, assuming they implement the Authenticatable contract.

Tests:

   PASS  Tests\Unit\UnitTest
  ✓ it is true                                                                                                             0.02s

   PASS  Tests\Feature\AuthKitLoginRequestTest
  ✓ it redirects to WorkOS without screen hint                                                                             0.26s
  ✓ it redirects to WorkOS with sign-in screen hint                                                                        0.01s
  ✓ it redirects to WorkOS with sign-up screen hint                                                                        0.01s
  ✓ it stores state in session                                                                                             0.01s
  ✓ it includes state in the authorization URL                                                                             0.01s
  ✓ it passes all parameters correctly to getAuthorizationUrl                                                              0.05s
  ✓ it supports domain hint parameter                                                                                      0.01s
  ✓ it supports login hint parameter                                                                                       0.02s
  ✓ it uses default redirect URL when not specified                                                                        0.03s
  ✓ it uses custom redirect URL when specified                                                                             0.01s
  ✓ it supports multiple parameters at once                                                                                0.01s

   PASS  Tests\Feature\FeatureTest
  ✓ example                                                                                                                0.02s

  Tests:    13 passed (39 assertions)
  Duration: 0.66s